Blood samples were drawn from an internal iliac artery and subcutaneous abdominal vein of 3 Holstein cows producing about 20 Kg of milk daily to measure amino acid uptake by the lactating mammary gland. Of the essential amino acids, uptakes of Arg, Val, Ileu, and Leu were in excess of their outputs in milk protein. Many of the nonessential amino acids were taken up in insufficient quantities to account for their outputs in milk protein. Mammary tissue incubation studies with L(U-14C) branched chain amino acids revealed the ability of this tissue to catabolize Val, Ileu, and Leu. From the intermediates identified, it was determined that the pathways of degradation of Val, Leu, and Ileu by mammary tissue are those common to other tissue. Small but significant amounts of radioactivity were recovered in Glu, Asp, and specific organic acids known to be catabolic products of each amino acid, suggesting that catabolism of the excess balance of these amino acids represents potential sources of carbon and alpha-amino nitrogen for nonessential amino acid synthesis. Fifty Holstein cows were used in a 310-day lactating study to determine the nutritional value of urea versus soybean meal as sources of supplemental nitrogen in 5 corn based diets that contained 9 to 14.5% crude protein. Production of milk, 4% FCM, milk protein and milk fat was greatest when the percentage of crude protein in the diet was highest. Cows fed diets that contained 11-12% crude protein, with supplemental nitrogen supplied either as urea or soybean meal, produced similar quantities of milk, milk protein and milk fat. However, cows fed a diet that contained 13.5 to 14.5% crude protein with supplemental nitrogen supplied as soybean meal produced more milk and milk protein than cows fed a similar diet that contained urea. The increased production from feeding soybean meal resulted in nitrogen from soybean meal being utilized more efficiently for milk production than nitrogen supplied as urea when the diet contained 13.5-14.5% crude protein. These results suggest that urea may have limited value in the diet of the high producing dairy cow during early lactation since the degradation of dietary protein in typical dairy cattle diets would supply adequate ammonia for maximum microbial protein synthesis.